The antigen receptors on lymphocytes play pivotal roles in controlling the balance between tolerance and immunity. In B cells, the B cell antigen receptor (BCR) transmits signals that positively or negatively regulate lymphocyte survival, growth, and differentiation. B cell activation can be modulated by co-receptors, proteins on the cell surface that positively or negatively influence the threshold for BCR activation. The long-term goals of the proposed research are 2-fold: to use synthetic ligands to investigate how the organization of proteins on the B cell surface influences signaling and to develop a general strategy to modulate specifically the output responses of B cell clones. The proposed research is divided into 4 aims. Aim 1 is to synthesize multivalent ligands that vary in valency and can activate signaling through the BCR. The goal of this aim is to address the questions: Does the extent of BCR clustering influence B cell activation? Does the extent of clustering influence the localization of the BCR into lipid rafts? Aim 2 is focused on investigating the effects of co-clustering the positive regulatory receptor CD19/CD21 complex with the BCR. We propose to evaluate how a ligand's structure influences its ability to potently activate positive B cell responses. Aim 3 is to compare the effects of co-clustering negative regulatory co-receptors and the BCR. These studies will examine the relative ability of the inhibitory receptors CD22 and FcgammaRIIb, which negatively regulate B cell activation through the recruitment of different phosphatases, to influence cellular responses. Aim 4 is directed at testing the effects of the multivalent ligands on primary B-cells in vitro and in vivo. The goal of this aim is to examine how different populations of primary B cells respond to potent BCR modulators identified in aims 1-3 and to investigate whether these synthetic ligands can be used to manipulate primary B cell populations. We anticipate that our studies will provide insight into the signaling processes and may lead to new strategies for the generation of vaccines and/or compounds that inhibit autoimmune responses.